Nationals starter Ross Detwiler threw a shortened bullpen session this afternoon that left Manager Davey Johnson pessimistic about the left-hander making a quick return from the lower back strain that landed him on the disabled list before the all-star break.

Johnson ruled out Detwiler starting Friday in a doubleheader against the Mets, and he does not think Detwiler will be able to begin a rehab assignment before the Nationals’ current homestand ends July 28.

“He’s still not quite right,” Johnson said.

Detwiler’s continued lack of progress may push the Nationals to trade for a starting pitcher before the July 31 non-waiver trade deadline. Some rival executives have expected the Nationals to seek a starting pitcher since Detwiler went on the DL on July 7.

Rizzo, though, said he needs to wait longer to evaluate Detwiler before he can make a decision on dealing for another starter.

“I want to make sure that I know where Detwiler is at, because that’s the most important thing,” Rizzo said. “If Detwiler is healthy, that helps me make any and all decisions we have to make.”

If the Nationals decide to pursue a starter, some close to the organization believe they may target veteran White Sox right-hander Jake Peavy. Peavy returned from the disabled list today and allowed the Braves two runs over six innings. Peavey, 32, last started June 4 before his strong outing today.

Peavey would fulfill many of Rizzo’s preferences. He has experience, having pitched 12 major league seasons. Rizzo eschews trading young talent for rental players, and Peavy is under contract for this year and 2014. This winter before hitting free agency, Peavy signed a two-year extension that pays him $14.5 million per year. He also owns a player option for 2015.

Again, Rizzo said he wants to wait for a better read on Detwiler before making any additions to the rotation. (The Nationals also have to consider an innings limit on talented rookie Taylor Jordan and Dan Haren’s struggles in the first half.)

Rizzo said Detwiler told him he felt “good” after his throwing session. Johnson had a gloomier outlook. He expected Detwiler would be available coming out of the all-star break. Instead, after Detwiler’s session today, Johnson believes it could be almost two weeks before he’s able to throw in a minor league game.

Detwiler threw roughly 25 pitches off the mound as pitching coach Steve McCatty and two team athletic trainers watched. Detwiler could not throw with full extension and will have to wait another few days before throwing off a mound again, Johnson said.

“I’m sure that is something we’re concerned about,” Johnson said. “We’re going to definitely have him not rush back out, force him, push him. He’s a gutty young man. He’d take the ball and go out there the way he is. But we want him to have no discomfort. He’s got too great a future.”

Injuries have derailed a once-promising season for Detwiler. In his first seven starts, Detwiler punched up a 2.53 ERA. In his next appearance, he strained his right oblique against the Dodgers. After he came back from the disabled list, Detwiler went 0-3 with a 6.31 ERA over five starts before he felt a strain in the left side of lower back.